1. Field of the Invention
The present invention relates to a gear made of a resin, which is used widely in a power-transmitting mechanism for a duplicator, a printer, facsimile, an automobile part.
2. Description of the Related Art
A gear made of a resin is conventionally used in a power-transmitting mechanism for a duplicator, an automobile part and the like for the purpose of reducing the part cost and the weight and the operational sound. The gear made of the resin is formed into a predetermined lightened shape by an injection molding, but the deformation such as a warpage and a sink may occur due to a difference between the amounts of material shrunk at molded portions. For example, as shown in FIGS. 1 to 3, in a gear 1 formed of a resin so that a boss 3 and a rim 5 are connected to each other by a web 4, the amount of shrinkage or contraction of a connection 7 between the rim and the web 4 and the amount of shrinkage of an end of the rim 5 are different from each other and for this reason, there is a possibility that a sink (indicated by a broken line L1) is produced, i.e., the connection 7 between the rim 5 and the web 4 is deformed so that it is recessed. If such a sink is produced, the tooth flank accuracy is degraded. In a gear 1 formed of a resin with a web 4 offset toward one of ends of a rim 5, as shown in FIG. 3, the following disadvantage is encountered: A connection 7 between the web 4 extending radially outwards from an end of the boss 3 and the rim 5 is shrunk in a large amount and as a result, the free end of the rim 5a is deformed, namely, warped (indicated by a broken line L2) to look like being expanded. Particularly, in a gear having a large tooth width as in the prior art shown in FIG. 3, the above disadvantage is significant.
In order to prevent the advantage associated with such gear 1, various gears made of a resin are conventionally proposed, which include ribs extending radially between a boss and a rim 5, or a annular rib, and a web 4 formed at a smaller thickness, while ensuring the strength, so that the deformation such as a warpage and a sink is inhibited by regulating the thickness at each of various portions, thereby enhancing the tooth flank accuracy (for example, see Japanese Patent Application Laid-open No. 9-230657 and the like).
In the conventional gears made of the resin, however, a predetermined tooth flank accuracy is provided, but the demand for an increase in accuracy for the gear made of the resin is considered to be more and more intensified. Therefore, it has been desired in industries for manufacturing a duplicator and an automobile part to provide a gear made of a resin, wherein the deformation such as the warpage and the sink can be inhibited effectively to further enhance the tooth flank accuracy.